The field of the invention is nuclear magnetic resonance methods and systems. More particularly, the invention relates to an RF synthesizer and transmitter for producing RF excitation pulses having a precise frequency and phase, and a receiver for accurately receiving and digitizing the resulting NMR signals.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomenon are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.o), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped, and hence, the magnitude of the net transverse magnetic moment M.sub.t depends primarily on the length of time and magnitude of the applied excitation field B.sub.1 and its frequency.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation signal B.sub.1 is terminated. In simple systems the excited nuclei induce an oscillating sine wave signal in a receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.t. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.0 e.sup.-t/T*.sub.2
The decay constant 1/T*hd 2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the signal decays, at least in part due to dephasing of the aligned precession of the spins after removal of the excitation signal B.sub.1 in a perfectly homogeneous field.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest. If the net magnetic moment M is not given sufficient time to relax to its equilibrium value, the amplitude A of the NMR signal produced in a subsequent pulse sequence will be reduced.
The NMR measurements of particular relevance to the present invention are called "pulsed NMR measurements". Such NMR measurements are divided into a period of RF excitation and a period of signal emission and acquisition. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more RF excitation pulses (B.sub.1) of varying magnitude, frequency content, phase and duration. Such RF excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which can produce transverse magnetization M.sub.t over a range of resonant frequencies. The prior art is replete with RF excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
More recently NMR techniques have been developed which place additional stringent requirements on the RF transmitters. Some methods such as phase spoiled steady state sequences require that the phase of successive RF excitation pulses be shifted by programmed amounts, and other methods such as the use of fast passage inversion pulses require that the RF excitation pulses be phase modulated by a predefined waveform. Still other methods such as multi-planar imaging require that the carrier frequency of successive RF excitation pulses be changed in a programmed pattern, and still other methods which use variable rate excitation pulses require that the RF excitation pulse be frequency modulated. Other methods, such as offset field of view imaging, require that the frequency of the reference signal used to demodulate the received NMR signal be frequency offset with respect to the Larmor frequency, or phase offset by programmed amounts from sequence to sequence. All of these methods have the common requirement that the relative phase of the carrier signal used to produce the RF excitation have a consistent or known phase relationship to the reference signal used to demodulate the received NMR signals. If the synthesizer signals used in the multiple sequences that generate an NMR data set do not have this phase consistency, the quality of the data will be degraded. Prior synthesizers are not able to provide this versatility while maintaining phase consistency, or can do so only with great inconvenience.